In the technical development of structures having a hierarchical structure, in which an organic compound and an inorganic compound are highly composited, a biomimetic approach has been particularly focused upon. In biological systems, a mineral compound (for example, metal oxide such as silica) is orientated along the organizational structure of a biological polymer (for example, protein, and polyamine) to form a sophisticated organization of an organic-inorganic composite. Therefore, a deposition method involving silica, titania, etc. using an organic polymer, in particular, basic polyamines, brings new possibilities for making the organic-inorganic composite having a hierarchical structure. Due to this, this method has received much attention from material science technical fields.
For example, the present inventors, etc. have found that a nanostructure containing metal oxide having various complex shapes, such as a nano-fiber is produced using a special molecular aggregate which is generated by a linear polyethyleneimine made of secondary amine in an aqueous solvent as a participation field of the metal oxide, and the process for producing the same (For example Patent Documents Nos. 1 to 4). After the polymers having the linear polyethyleneimine skeleton are spontaneously aggregated in water, and a source solution of the metal oxide is mixed into the solution containing the aggregates, the metal oxide is condensed selectively on the surface of the polymer aggregates hydrolytically. Due to this, the nanostructure, in which the polymer and the metal oxide are composited, is produced. The present inventors, etc have made the invention by finding these phenomena. This process is very effective for controlling the structure of the nanostructure in which an organic compound and an inorganic compound are highly composited. However, this process is only capable of depositing allowing the nanostructure composite to deposit in a solution.
On the other hand, as a method for producing a silica film on the surface of a solid material, a method for imitating biological silica, such as diatoms has been examined. Basically, this method is a method in which polyamines which work as a catalyst are fixed on the surface of a substrate by adsorption or chemical bonding, and silica is deposited generates thereon. For example, it has been reported that a molecular residue capable of starting a radical polymerization is fixed on the surface of gold, a radical polymerizable monomer having an amino group (for example, N,N-dimethylaminoethyl methacrylate) is radically polymerized with the molecular residue, the obtained plural poly(N,N-dimethylaminoethyl methacrylate) are formed on the surface of the gold such that they stand like bristles on a brush, and alkoxysilane is hydrolyzed and condensed on the amine polymer brush to form a film made of a composite of silica and a polymer (for example, Non-Patent Document No. 1). The surface of the obtained composite film is not flat. The surface is uneven and is made by arranging randomly silica particles, and is not made of dedicated patterns.
In addition, for example, it has also been reported that when poly(L-lysine) is used as polyamine, a copper plate is used as an anode, indium tin oxide (ITO) in a plate shape is used as a cathode, an aqueous poly(L-lysine) solution is added between the anode and the cathode, and an electrical field is applied, the poly(L-lysine) is adsorbed onto the ITO, and the ITO is immersed into a silicic acid solution, and silica generates on the surface of the ITO (for example, Non-Patent Document No. 2). The silica on the surface of the ITO which is produced by this method has basically a flake shape. However, a uniform film cannot be obtained. In other words, only a specific portion has a dense flake structure, and there are partial flakes of silica on the surface of the ITO in total. Furthermore, it has been reported that a glass stick is immersed into a basic polymer solution, such as poly(L-lysine), poly(L-lysine-tyrosine), and poly(arylamine), the basic polymer is adsorbed onto the surface of the glass stick, the glass stick is immersed into an aqueous silicic acid solution, and thereby silica can deposited on the surface of the glass stick (for example, Non-Patent Document No. 3). There is a grainy silica film on the surface of the glass stick. However, there are no characteristics for showing an existence of a nanostructure composite, and the glass stick looks like it is coated with silica.
A technique for easily producing a titanium oxide film, in which titania is focused as metal oxide, a titanium oxide source in a solution is deposited on the surface of a solid, has been developed (for example, Non-Patent Documents Nos. 4 to 6). This method is a method in which a so-called self-assembled monolayer (SAMs) is generally produced on the surface of a solid substrate, and the substrate is dipped into a titanium source solution, thereby a titanium oxide film is produced through a process of adsorbing titanium oxide crystals onto the SAMs. In this method, a plastic substrate or a silicone wafer can be used as the solid substrate. However, in both substrates, it is necessary to plant closely chemical function groups, such as —SO3H, —COOH, —OH, and —NH, on the surface of the solid substrates. These functional groups promote the crystal growth of the titanium oxide at nano-meter scale as a crystal core, and result in forming a continuous film made of titanium oxide crystals in nano-meter scale.
The titanium oxide film produced by these methods is absolutely a continuous film made of titanium oxide crystals themselves, and is not a film in which titanium oxide is highly combined with an organic compound. In addition, these methods cannot form a film of which the complex hierarchical structure is controlled programmatically. The SAMs on the surface of the solid substrate only work to fix the inorganic crystals, and do not exert other additional functions.
As explained above, although it is possible to fix polyamines on the surface of the substrate, and deposite silica or titania, it has not been possible to cover uniformly the substrate with a nanostructure composite having a controlled structure.
Since polyamines have basic properties, polyamines works as a catalyst in various reactions. When polyamines are used as a catalyst, polyamines have been used as a solid catalyst in view of separation from products or reuse thereof. For example, various solid catalysts, such as a catalyst in which polyamine or an organic basic compound is fixed on the surface of silica with a chemical bond (for example, Non-Patent Documents, Nos. 7 and 8), a solid catalyst in which an amine residue is fixed into mesoporous silica (for example, Non-Patent Document, No. 9), have been suggested.
The solid catalyst in Non-Patent Documents, Nos. 7 to 9 is characterized by chemically bonding a part of the compound having catalytic functions with the surface of the solid. Therefore, when the solid catalyst is reused, the compound having catalytic functions fixed on the surface of the solid easily changes its structure. Therefore, the catalytic activity is always decreased, and it is necessary to increase the amount of the solid catalyst used. Due to this, in general, it is difficult to use the basic solid catalyst disclosed in Non-Patent Documents, Nos. 7 to 9, etc, in industrial production.
In contrast to fixing a catalyst on the surface of a solid, a method in which molecules of the compound having catalytic functions are put in a polymer capsule has been suggested (for example, Non-Patent Document, No. 10). The catalyst produced by this method does not decrease its catalytic activity when it is reused. However, when it is used repeatedly, it is not simple to recover, compared with the solid catalyst.
When basic polyamines are used as a catalyst, if the catalyst works as a molecular catalyst, and is kept as a solid, it can be anticipated that the catalyst provides various benefits, such as improvement of catalytic activity, simplicity of separation and recovery, improvement of reuse efficiency, etc. These benefits bring advantages such as a decrease of environmental load, cost of production, etc. The most ideal catalyst is an immobilized catalyst type reactor in which a composition having catalytic functions is fixed to a reaction vessel or a reaction tube in nano-meter scale, which is expected to have superior catalytic efficiency because of having large specific surface area, and after reaction, the reaction solution is removed, and new reaction material can be put into the reaction vessel or the reaction tube. However, such reactors have not been found yet.
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